Lipids are an essential constituent of cell membranes and are sources of energy. Lipid mediator generation can be entirely innate or composite via an adaptive immune step. This chapter addresses only primary innate or exogenous signals assessed in vitro and in vivo. In vivo discussion is limited to studies in null mouse strains for clarity. The activation of phospholipase A2 (PLA2) with release of arachidonic acid provides the substrate for all eicosanoids, namely, prostanoids and leukotrienes (LTs). Although certain residual lysophospholipids by acetylation become platelet-activating factor (PAF), other lysophospholipids are generated by more complex pathways. PLA2 comprises a diverse family of enzymes that cleave the sn-2 position of glycerophospholipids to form a fatty acid and a lysophospolipid. PLA2 is involved in the digestion of phospholipids in the diet and in the metabolism and turnover of phospholipids in cell membranes. Lysophosphatidic acid (LPA) and S1P are bioactive lysophospholipids with cell functions that include growth, inhibition of apoptosis, differentiation, migration, and cytoskeletal rearrangement. The major biosynthetic pathway of LPA is initiated by the action of phospholipase D on phospholipids to form phosphatidic acid and then the cleavage of the sn-2 position of the fatty acid by phosphatidic acid-specific PLA2 to form LPA. Two lysophospholipds containing lysophosphorylcholine, namely, sphingosylphosphorylcholine (SPC) and lysophosphatidylcholine (LPC), are involved in many biological processes including cell proliferation and growth inhibition.